树的分治学了好几天时间也不是很透彻，特别是09年OI论文还没看懂。。。。。
这题很经典的一道树的分治
可以参考09漆神OI论文
1 #include <cstdio> 2 #include <algorithm> 3 #include <vector> 4 #include <cstring> 5 #include <iostream> 6 using namespace std; 7 #define N 10010 8 struct node { 9 int to, w; 10 node() {}; 11 node(int _v, int _l): to(_v), w(_l) {}; 12 }; 13 vector<node> g[N]; 14 int n, k, sizee, s[N], f[N], root, d[N], K, ans; 15 vector<int>dep; 16 bool vis[N]; 17 void getroot(int now, int fa) { 18 s[now] = 1; f[now] = 0; 19 int u; 20 for (int i=0; i<g[now].size(); i++) 21 if ((u = g[now][i].to) != fa && !vis[u]) { 22 getroot(u, now); 23 s[now] += s[u]; 24 f[now] = max(f[now], s[u]); 25 } 26 f[now] = max(f[now], sizee-s[now]); 27 if (f[now] < f[root]) root = now; 28 } 29 void getdep(int now, int fa) { 30 dep.push_back(d[now]); 31 int u; 32 for (int i=0; i<g[now].size(); i++) 33 if ((u = g[now][i].to) != fa && !vis[u]) { 34 d[u] = d[now] + g[now][i].w; 35 getdep(u, now); 36 } 37 } 38 int calc(int now, int init) { 39 dep.clear(); d[now] = init; 40 getdep(now, 0); 41 sort(dep.begin(), dep.end()); 42 int ret = 0; 43 for (int l=0, r=dep.size()-1; l<r; ) 44 if (dep[l] + dep[r] <= K) ret += r-l++; 45 else r--; 46 return ret; 47 } 48 void work(int now) { 49 int u; 50 ans += calc(now, 0); 51 vis[now] = true; 52 for (int i=0; i<g[now].size(); i++) 53 if (!vis[u = g[now][i].to]) { 54 ans -= calc(u, g[now][i].w); 55 f[0] = sizee = s[u]; 56 getroot(u, root=0); 57 work(root); 58 } 59 } 60 int main() { 61 while (scanf("%d%d", &n, &K) == 2) { 62 if (n == 0 && K == 0) break; 63 for (int i=0; i<=n; i++) g[i].clear(); 64 memset(vis, false, sizeof(vis)); 65 66 int u, v, l; 67 for (int i=1; i<n; i++) { 68 scanf("%d%d%d", &u, &v, &l); 69 g[u].push_back(node(v, l)); 70 g[v].push_back(node(u, l)); 71 } 72 f[0] = sizee = n; 73 getroot(1, root=0); 74 ans = 0; 75 work(root); 76 printf("%d ", ans); 77 } 78 return 0; 79 }